This invention relates to a plastic optical fiber low in light loss in the visible light region to near infrared region and excellent in heat resistance, a process for producing the same, and a transparent resin having a high refractive index and a low chromatic aberration usable for such a plastic optical fiber.
Recently plastic optical fibers have been noticed as fibers for transmitting information in cars and buildings due to their advantages in having a larger core diameter or aperture and flexibility compared with glass optical fibers. Heretofore, polystyrene and acrylic resins such as poly(methyl methacrylate) and the like synthetic resin materials excellent in transparency have been used as the core material in plastic optical fibers. But these materials had a defect in that the decay in light transmission through the fiber is large compared with the glass-made fiber. Thus, there have been made various proposals to reduce the light loss in plastic optical fibers.
According to an article disclosed in Japanese Journal of Applied Physics vol. 24, No. 12, pp. 1661-1665 (1985), major factors for the light loss during transmission in the visible light region to near infrared region in plastic optical fibers are an absorption loss caused by high harmonics in infrared vibration absorption between the carbon to hydrogen bond and the Rayleigh scattering loss caused by fluctuation in the density or concentration. In order to reduce the absorption loss, it is proposed to use a plastic optical fiber having a core made from a polymeric material containing heavy hydrogens (Japanese Patent Unexamined Publication No. 54-65536). It is also proposed to introduce carbonfluorine bonds into polymeric materials in order to reduce the vibration absorption of carbon-hydrogen bonds as well as the Rayleigh scattering (Appl. Phys. Lett. 48(12), pp. 757-758, 24 Mar., 1986). But since the polymeric materials used as the core materials in these plastic optical fibers are so-called polymethacrylate esters introducing carbon-heavy hydrogen bonds or carbon-fluorine bonds in place of carbon-hydrogen bonds, the heat resistance is as low as 70.degree. to 80.degree. C. at most. Further, since the glass transition temperature (Tg) of fluoroalkyl methacrylate polymers or fluoroalkyl acrylate polymers is 75.degree. C. or lower, the heat resistance is further lowered. In addition, it is known that since methyl methacrylate polymers containing heavy hydrogens easily absorb moisture, the light loss becomes large. As mentioned above, the plastic optical fibers had a defect in that initial properties were easily deteriorated by changes of circumstances.
On the other hand, it is an effective method to introduce heavy metal atoms into plastics for making the refractive index of plastics higher. Techniques relating to metal-containing plastic lens materials are disclosed, for example, in U.S. Pat. Nos. 3,920,605 and 3,886,125. But monomers having covalent bonds with metals or metal salts of carboxylic acids having double bonds are generally poor in solubility in styrene or a styrene derivative. Further, even if only one kind of these metal salts of carboxylic acids is copolymerized with a reactive monomer such as chlorostyrene, the resulting resin is poor in transparency and still has a problem as a material for plastic lenses. In addition, in order to maintain the high transparency of the resin, it is necessary to remarkably reduce the amount of metal contained in the resin, which results in making it impossible to maintain the high refractive index of the resin.